The present invention relates to an apparatus for transmitting a digital signal.
When a digital signal is transmitted optically or recorded and reproduced magnetically, there are produced nonlinear distortions in the signal transmitted by the relative system. Specifically, in the case of magnetic recording and reproduction, it is known that the waveform of the reproduced digital signal, as the recording density is increased, becomes distorted due to nonlinearity of the recording and reproducing system, restriction by the transmission band of the recording and reproducing system, etc. and therefore the data cannot be reproduced accurately. Such a phenomenon is called a peak shift. The peak shift phenomenon is remarkably observed when the signal waveform is such that the logical levels "0" and "1" alternate therein after the same level of "0" or "1" has lasted long, i.e., after a long time run-length.
As one of the countermeasures to the peak shift phenomenon, a method has been proposed to add a complementary distortion to the recorded digital signal at the recording side in advance, estimating the distortion to be produced in the recording and reproducing system. This compensating method employs an analog phase modulating circuit consisting of a capacitor and a resistor connected to advance the phase of the leading edge in a digital signal following a long run-length.
However, since the above mentioned peak shift compensating method makes use of an analog phase modulating circuit, there are difficult problems in keeping specified conditions against changes in the temperature and fluctuations of the supply voltage and also designing, adjusting or the like of the circuits becomes complex.
On the other hand, there are known methods to suppress the occurrence of the peak shift by modulating a digital signal such that the run-length may be decreased, such as, for example, the PE (phase encoding) method and the M.sup.2 method. In such modulating methods, however, the volume of the data to be transmitted is increased over that of the original data and their transmission efficiency is lowered.